Method and device for guiding cast billets in a continuous casting facility

ABSTRACT

The invention relates to a method for guiding strands in a continuous casting installation, in particular an installation for producing thin slabs made of steel, having an ingot mold and a strand-guidance frame which is provided with a cooling device. In this case, heat is extracted indirectly from the strand after it leaves the ingot mold, and the strand, at least in sections, is held in shape, guided in the strand discharge direction and additionally cooled by means of a gaseous medium. Furthermore, the strand is additionally moved in a defined manner and at a predeterminable speed through the strand frame by mechanical means, the speed of the strand being accelerated or decelerated.  
     The invention furthermore relates to a device for carrying out the method. See FIG. 1.

DESCRIPTION

[0001] The invention relates to a method for guiding strands in a continuous casting installation, in particular installations for producing thin slabs made from steel, having a stationary ingot mold or a moving ingot mold and a strand-guidance frame which is provided with a cooling device, and also to a device for carrying out the method.

[0002] As a rule, strand guidance in continuous casting installations for slabs or blooms is carried out by means of rolls arranged below the ingot mold. If they are uncooled, these rolls have a minimum diameter of approximately 100 mm, while if they are internally cooled they have a minimum diameter of approximately 140 mm. In the case of continuous slab casters, which are able to accommodate a slab width of up to 3.5 m, split rolls with intermediate bearings are used.

[0003] As a rule, when uncooled rolls are used a spray cooling system is employed. This type of spray cooling system presents a risk of uncontrolled cooling of the strand, which can lead to surface cracks on the strand.

[0004] The diameter of the rolls, in conjunction with the strand width, determines the distance between the individual rolls. This roll-to-roll distance, which can be regarded as the characteristic variable for the strand support or the strand bulging, has a direct effect on the quality of the strand. The bulging of a strand is dependent on the casting speed and on the distance between the rolls. While standard slabs of a thickness of approximately 200 mm are cast at a maximum speed of 2.2 m/min, thin slabs with a thickness of approximately 50 mm are cast at a speed of 6 m/min, and speeds of 8 m/min are being striven for.

[0005] An aggravating factor is that the strand shell of thin slabs is considerably hotter, right from leaving the ingot mold until completely frozen, by comparison with strand shells of standard slabs at the same metallurgical location.

[0006] Since the diameter of the rolls and also the distance between the individual rolls cannot be reduced arbitrarily, the tendency toward bulging and hence toward deformation of the strand increases in a manner which can no longer be controlled as casting rates become higher and at the same time the cast thickness falls.

[0007] In addition to the strand-guidance rolls, plates are also known as strand-guidance components. For example, EP-0,107,563 A1 proposes a grid network which is arranged below the ingot mold and through the free space in which a cooling medium, for example spray water, is sprayed against the surface of the strand.

[0008] The drawbacks of these components consist in the fact that high frictional forces occur between the strand and the plates, and moreover there is a risk of quality drawbacks in the form of breakouts and also of deflagration caused by included water. Furthermore, high withdrawal forces are required for withdrawing the strand, leading to a high load on the strand shell.

[0009] The object of the invention is therefore to use simple means in order to provide strand guidance even for high casting speeds allowing the production of strands of high surface quality with a low level of wear and employing means of simple designs.

[0010] The invention achieves this object by means of the characterizing features of method claim 1 and device claim 5.

[0011] According to the invention, after leaving the ingot mold the strand slides over a cushion of gas which is positioned between plate segments and the strand, and its heat is extracted indirectly due to the fact that the radiant heat is taken up by cooling plates which are not in contact with the strand. The gaseous medium used is preferably nitrogen, which by means of a suitable design of the plate segments on the wall facing the strand holds the latter in shape, guides it in the strand discharge direction and also the flowing gas cools the strand in addition to the cooling plates taking up the indirect heat (radiation).

[0012] In a curved installation, the strand is curved through a constant curve over a plurality of bending points or else continuously. The work which is to be done in each case by the gas film from the segment ∘ and the following segment comprises:

[0013] supporting the strand as a function of the weight proportion in a corresponding manner from the location of strand guidance between the vertical and horizontal parts of the cast strand guidance

[0014] compensating for the ferrostatic pressure as a function of the vertical distance to the casting level

[0015] bending and straightening of the strand

[0016] rolling and casting, reduction in the strand thickness during freezing

[0017] conveying of the strand.

[0018] It should also be taken into account that the strand guidance plate segments do not have to do any work to support (the weight of) the strand on the top side. Moreover, the specific amounts of work vary in the respective segments and over the metallurgical length from the ingot mold outlet to the end of strand guidance.

[0019] The different amounts of work per plate segment are ensured using segment-specific design and/or control of the gaseous medium in pressure times volume. Particular account is taken here of the dependency of the current shell thickness of the strand. When designing the installation, attention is paid to setting the level of work done pneumatically on the strand such that the strand is guided, conveyed and—if desired—reduced in terms of its cast thickness but is not deformed in an uncontrolled manner by indentation, that is to say negative bulging.

[0020] At the same time, the strand is additionally moved in a defined manner and at a predeterminable speed through the strand frame by mechanical means (in this case essentially continuous casting rolls). The rolls can assist with, perform and ensure the transportation and/or the desired casting speed and/or the bending and straightening processes. The roll arranged at the end of the continuous casting frame is used to ensure the casting speed in a defined manner, since by this stage the strand has completely frozen.

[0021] The drive powers introduced via the gas stream and the continuous casting rolls can be matched to one another as desired. In an advantageous configuration of the invention, the strand is conveyed by the gas stream and its speed is decelerated to the desired speed by means of the continuous casting rolls.

[0022] The plate segments essentially comprise a hollow body through which a cooling medium is guided, preferably with suction. A network of distribution lines, through which a gas, for example nitrogen, is guided, is provided in the plate segments on the side inclined toward the strand. In sections, the distribution lines are connected to one another and via collection lines are connected to a gas-conveying station. The individual nozzle orifices of the distribution lines may be formed differently. Their distribution may also be adapted in accordance with the work done at their location in the strand guidance. Thus it is possible, for example, for the number of nozzles and/or the sum of the nozzle orifices in the segment to be changed in functional terms over a metallurgical length and width, in order to do different amounts of work at different geometric locations while supplying the same pressure. This distribution of the nozzles in the sense of different outputs at constant pressure, e.g. per pressure system (segment plate, pneumatic cushion), can be present both transversely and longitudinally with respect to the casting direction. It is also possible for a segment to be connected to different, mutually independent pneumatic systems.

[0023] Furthermore, it is proposed to provide the nozzles or some of the nozzles with an acute angle of inclination with respect to the casting direction, in order to assist with conveying the strand. This assistance with strand conveying fixes the continuous casting speed and assists with and simplifies the work done by the pairs of rolls arranged between the plate segments. At least in the edge region, the plate segments are at a distance from the strand surface allowing defined leakage of the gas between the strand and the lip arranged at least in the edge region.

[0024] The plate segments are connected to actuators, for example piston-cylinder units, by means of which the distance between plate segment and strand, and hence the gas leakage, can be set in a predeterminable manner.

[0025] The wall which is inclined toward the strand is designed in terms of its form and material in such a manner that it is able to dissipate the maximum possible amount of radiant heat. It is preferred here to use copper of a relatively low wall thickness, in order to take up the heat by means of the cooling water arranged in the cooling chamber of the plate segment.

[0026] In a further refinement of the invention, the wall thickness of that wall of the plate segments which is inclined toward the strand is varied, specifically in such a manner that the thickness decreases towards the principal axis of the plate. In addition, that wall of the segment which faces the strand may be coated with a wear-resistant protective layer, such as for example nickel and/or chromium.

[0027] In one refinement, the plate segment is composed of pipes, which are arranged in a meandering form, the distribution lines for supplying the gas being introduced at the lines where these pipes are in contact with one another. The pipeline makes it possible to convey cooling water at high speed and hence to dissipate as much radiant heat as possible from the strand.

[0028] In order to guarantee that the strand runs centrally, it is proposed for that wall of the plate segments which is inclined toward the strand to be designed concavely in the direction of travel of the strand. The extent of the concavity of the wall may in this case begin at a concave ingot mold and be reduced in the first segment over all the segments of the strand guidance, for example to a low level of concavity or even a planar surface by the end of the strand-guidance frame or by the point at which the strand has completely frozen. In this way, it is possible to produce a parallel slab or a slab having a desired concave section.

[0029] Furthermore, the profile of the work performed pneumatically over the segment width can be predetermined, for example by means of different hole thicknesses. Thus it is advantageous, for example, to build up a lower level of pneumatic work in the center of the plate segment, in order to cope with the membrane effect of the strand in its center. At the edge of the strand, i.e. in the peripheral region, the strand is more dimensionally stable than in its center.

[0030] Moreover, for predetermined strip thicknesses the amount of work done pneumatically can be reduced or increased for the same gas pressure by opening or closing the plate segments, i.e. by varying the distance between the plate segments and the strand. This level of work done pneumatically can be predetermined by means of the distance of the plate segments on the top and bottom sides, given a predetermined slab thickness.

[0031] Essentially, in the device according to the invention the strand is conveyed pneumatically through the continuous casting machine at a desired casting speed. The motors of the continuous casting rolls ensure the precise desired casting speed by means of additional work, either conveyance (motor operation) or the rolls run in generator mode and decelerate the slab to the desired casting speed. If the motor current rises above predetermined limits, the basic speed is corrected pneumatically.

[0032] The proposed method and device achieve the following results:

[0033] the strand is not subject to any bulging and hence deformation, even at high casting speeds of up to 10 m/min.

[0034] the strand does not require any direct water cooling and hence exhibits minimal energy loss,

[0035] if inert gas is used, scaling of the strand is avoided,

[0036] progressive bending and straightening with the lowest possible area-specific deformation is possible,

[0037] progressive casting and rolling, and also a reduction in the thickness of the strand during its freezing, are possible

[0038] there are no rotating mechanical components, which has the advantage that:

[0039] consequently wear to the machine is minimal, and

[0040] a high casting reliability is achieved by comparison with the so-called grids which are cooled with spray water, and

[0041] there are no mechanical limits on the conveyance of the strand, as is the case, for example, with continuous casting strand-guidance frames comprising rolls, particularly in the case of very wide, quick-casting slab installations, in particular thin slab installations.

[0042] An example of the invention is depicted in the attached drawing, in which:

[0043]FIG. 1 shows a schematic diagram of the continuous casting device

[0044]FIG. 2 shows a section through the continuous casting device

[0045]FIG. 3 shows plate segments with concave walls

[0046]FIG. 4 shows a plate segment comprising a pipe arranged in meandering form.

[0047]FIG. 1 shows a continuous casting device having a tundish 11, from which liquid metal is guided via an immersion pipe 13 into an ingot mold 12. In the present case, this is a curved continuous casting installation for producing slabs B, in which the strand is withdrawn, with its strand shell W surrounding the liquid crater S, from the ingot mold 12 which is aligned vertically. Beneath the ingot mold 12, there are provided rolls 21, which are driven by means of a roll drive 22, and a guidance system made of plates 30 and comprising individual plate segments 31. The plate segments 31 are arranged on the top and bottom sides of the slab B. The rolls 21 are arranged between the individual plate segments 31.

[0048] Furthermore, the roll drives 22 are connected to a measuring and control system 23.

[0049] The rolls 21 which guide and drive the strand, and also the plate segments 31, are surrounded by a housing 61. The housing 61 is connected to a gas cooling installation 64 and a gas purification installation 85 via lines 82 for supplying a gas and lines 63 for returning a gas. A gas-conveying station 49 is provided in the gas line 62, by means of which station the gas is conveyed to the individual plate segments 31 via gas collection lines 46.

[0050] To cool the individual plate segments 31, the cooling medium is extracted by a pump 71, via a feed line 72, fed to the individual plate segments 31 and returned via a return line 73.

[0051]FIG. 2 shows the section A-A through the continuous casting device with the housing 61 which surrounds the plate segments 31 and the slab B.

[0052] The plate segments 31 are attached adjustably to provide the spacing of the plates, by means of piston-cylinder devices 51.

[0053] A cooling medium is fed to the plate segments 31 via the line 72 and is returned via the line 73.

[0054] To supply the gas, gas is conveyed, via the collection line 46 and distribution lines 45 for the distribution in the outer region, via a distribution line 43 for supply in the central region and also distribution lines 44 for supply in the intermediate region, via mouths 42 into the space between the plate segments 31 and the slab B. The plate segments 31 lie on that side of the wall 32 which is inclined toward the slab and in the edge region may have a lip 34.

[0055] The slab B has the strand shell W, in which there is a liquid crater S of liquid metal.

[0056]FIG. 3 shows a further section A-A through the plate segments 31, in which the wall 32 inclined toward the strand B is of concave form. In the top part of the figure, the wall 32 is of double-walled design, the plate 35 which is directly assigned to the strand B being made of a material of high thermal conductivity, e.g. copper. The copper plate 35 may in this case be coated with a wear-resistant layer 36, for example a layer of nickel or chromium.

[0057] In the upper region of the figure, the wall 32 has a lip 34 in the edge region. In this case, the wall thickness d of the wall 32 may be designed to become thicker from the central region toward the edge region.

[0058] The gas is supplied to the mouths 42 via the distribution line 43 and bores 41.

[0059] The slab B has the strand shell W, which surrounds the liquid crater S. In the central region, the slab B has a bore.

[0060]FIG. 4 shows a plate segment 31 which comprises a pipe 38 which is arranged in meandering form and is connected to the feed line 72 and the return line 73. At the line of contact 39, the distribution lines 43, 44, 45 end directly on the side inclined toward the slab B.

[0061] The lower part of the figure shows the section B-B, in which the pipes are connected in a gastight manner at the line of contact 39 and are not in direct contact with one another only at the location where the distribution lines 43, 44, 45 pass through. The mouths 42 of the distribution lines point into the space between the plate segment 31 and the slab B.

LIST OF REFERENCE NUMERALS

[0062] Continuous Casting Ingot Mold

[0063]11 Tundish

[0064]12 Ingot mold

[0065]13 Immersion pipe

[0066] Strand-guidance Frame

[0067]21 Roll

[0068]22 Roll drive

[0069]23 Measuring and control system

[0070]30 Plates

[0071]31 Plate segment

[0072]32 Wall

[0073]33 Cushion: central region

[0074]34 Lip: edge region

[0075]35 Copper plate

[0076]36 Wear-resistant layer

[0077]38 Pipe arranged in meandering form

[0078]39 Line of contact

[0079] Gas Supply

[0080]41 Bore

[0081]42 Mouth

[0082]43 Central distribution line

[0083]44 Intermediate distribution line

[0084]45 Outer distribution line

[0085]46 Collection line feed

[0086]49 Gas-conveying station

[0087] Plate Distancing

[0088]51 Means, piston-cylinder device

[0089] Enclosure

[0090]61 Housing

[0091]62 Lines to

[0092]63 Lines back

[0093]64 Gas cooling installation

[0094]65 Purification installation

[0095] Water Cooling System

[0096]71 Pump

[0097]72 Feed line

[0098]73 Return line

[0099]74 Water collection reservoir

[0100] B Slab

[0101] S Liquid crater

[0102] W Strand shell

[0103] d Wall thickness 

1. A method for guiding strands in a continuous casting installation, in particular an installation for producing thin slabs made of steel, having an ingot mold and a strand-guidance frame which is provided with a cooling device, wherein heat is extracted indirectly from the strand after it leaves the ingot mold, wherein at least in sections the strand is held in shape guided in the direction of the strand discharge, and additionally cooled by means of a gaseous medium, wherein a stream of gas is directed onto the strand surface in such a way as to influence the strand speed, and wherein the strand is additionally moved in a defined manner and at a predeterminable speed through the strand frame by mechanical means, namely continuous casting rolls, the speed of the strand being accelerated or decelerated.
 2. The method as claimed in claim 1 , wherein the gaseous medium can be set in a predeterminable manner with regard to the volume and the pressure as a function of the shell thickness.
 3. The method as claimed in claim 1 , wherein the thickness of the strand at the ingot mold outlet is reduced in at least one plate segment.
 4. A continuous casting device for producing strands, in particular thin slabs made of steel, having an ingot mold, which is followed in the casting direction by a strand-guidance frame which has rolls bearing against the strand surface and plates provided with bores, and having components for cooling the strand surface, for carrying out the method as claimed in claim 1 , wherein the plates (30) are divided into segments (31), between which rolls (21) are provided, wherein a network of distribution lines (43, 44, 45), through which a gaseous medium can be supplied, is provided in the plate segments (31) on the side inclined toward the strand (B), wherein the distribution lines (43, 44, 45) are connected to one another in sections and via collection lines (46) are connected to a gas-conveying station (49) wherein the mouths (42) of the distribution lines (43, 44, 45) are arranged, at an angle which is inclined with respect to the direction of travel of the strand (B), in that wall (32) of the plate segment (31) which is inclined toward the strand (B), wherein the wall (32) which is inclined toward the strand (B) is constructed in terms of shape and material in such a manner that it is possible to dissipate the maximum possible amount of radiant heat from the strand shell (W), and wherein the plate segments (31), at least in the edge region (34), are arranged at a distance from the strand surface allowing a defined leakage of the gaseous medium.
 5. The continuous casting device as claimed in claim 4 , wherein a wear-resistant layer, e.g. nickel and/or chromium, is applied to the wall (32) which is inclined toward the strand (B).
 6. The continuous casting device as claimed in claim 3 , wherein the plate segments (31) are composed of pipes (38) which are arranged in meandering form and through which a cooling medium flows, with the distribution lines (43, 44, 45) provided perpendicular to the pipe guidance at the lines (39) where the pipes are in contact with one another.
 7. The continuous casting device as claimed in claim 4 , wherein means (51) are provided for independently adjusting the distance between the plates and the strand (B).
 8. The continuous casting device as claimed in claim 4 , wherein the plate segments (31) are designed as cushions (33) which, in the edge region, have a lip (34) which affects the leakage of the gaseous medium and also the dissipation of heat from the strand (B).
 9. The continuous casting device as claimed in claim 8 , wherein the wall thickness (d) of the wall (32) of the plate segments (31) decreases toward the principal axis of the plate.
 10. The continuous casting device as claimed in claim 4 , wherein that wall (32) of the plate segments (31) which is inclined toward the strand (B) is of concave configuration in the direction of travel of the strand (B).
 11. The continuous casting device as claimed in claim 10 , wherein the extent of the concavity of the wall (32) decreases in the direction of travel of the strand (B) and becomes zero at the end of the strand-guidance frame having the plate segments (31).
 12. The continuous casting device as claimed in claims 11 or 7, wherein the means (51) are pneumatic actuators which are provided at least in the first position of the first plate segment 31 downstream of the ingot mold (12).
 13. The strand-guidance frame as set forth in claim 4 , wherein the number of distribution lines (43, 44, 45) in the plate segment (31) can be varied functionally depending on the metallurgical length, taking into account the membrane effect of the strand shell thickness.
 14. The strand-guidance frame as set forth in claim 4 , wherein the internal diameter of the distribution lines (43, 44, 45) in the plate segment (31) is adapted in functional terms over its metallurgical length in order to do a predeterminable amount of work in accordance with its geometric location while supplying the same amount of pressure.
 15. The continuous casting device as claimed in claim 4 , wherein the rolls (21) arranged between the plate segments (31) are connected to an adjustable roll drive (22), by means of which their rotational speed and tensile force can be set.
 16. The continuous casting device as claimed in one of the preceding claims, wherein the plate segments (31), the rolls (21) and the strand (B) are surrounded by a housing (61), which via a system of lines (62, 63) is connected to a gas cooling (64) and purification installation (65).
 17. The continuous casting device as claimed in claim 16 , wherein the system of lines (62, 63) is designed in such a way that the gas, for example nitrogen, can be fed back to the compressor after being purified and cooled. 